Phase modulation communications is well known in the art. Phase modulation is a form of modulation that represents information to be conveyed as variations in the instantaneous phase of a carrier wave. Intersymbol interference (ISI) is a form of distortion of a signal whereby symbols transmitted before and after the current symbol in a close neighborhood to it have an effect on the currently received symbol. This is an unwanted phenomenon as the neighboring symbols contribute to a noise effect, making communications less reliable. Several approaches to fight against intersymbol interference include using adaptive equalization or error correcting codes (i.e. especially soft-decoding using the Viterbi algorithm).
ISI is usually caused by echoes or a nonlinear frequency response of the channel. It is introduced in wireless communication links using phase modulated schemes, by the channel or by the receiver implementation impairments. The ISI creates an error vector magnitude (EVM) floor which degrades overall performance of the demodulator. Note that in the case where the modulation is a differential like 8-DPSK, ISI creates the DEVM floor. In certain modulation schemes such as Bluetooth enhanced data rate (EDR), the receiver often utilizes a filtering scheme which filters the required channel before the demodulation step (after down conversion). The complexity of the filter determines the amount of ISI observed at its output. Thus, when a less complex filter (in terms of area and power) is in use, so further ISI will be introduced. Furthermore, in case there is a frequency offset of the down converted signal prior to the filter, the ISI is more severe since the filter is generally designed under the assumption of zero frequency offset, due to the fact that the frequency offset is unknown during the filter design phase. Any frequency offset in the timing recovery loop contributes to ISI as well.